More than 170 Iranian students have been forced to defer enrollment and turn down funding at US universities this year because of delays in visa processing.

The students were accepted into graduate programs to study subjects ranging from English to engineering, but as classes begin, their lives have been put on hold as they wait for the documents needed to travel to the US.

It is unclear why so many visa applications have been held up. Possible explanations offered by students, advocates and immigration experts included the Iran nuclear deal, the attempted coup in Turkey and a lack of Farsi speakers at US embassies.

Iranians face more obstacles in getting student visas because the US and Iran do not have diplomatic relations.

Afshin Ahmadi was supposed to have begun his third year studying electrical engineering at Clemson University in South Carolina last week, but his visa has not been approved.

“I can’t do anything,” Ahmadi said. “I just need to stay at home, and check the internet and see if I’m clear or not.”

He has spent the past two years studying in the US and applied for a new visa after he made his first trip back home this summer. But 85 days after his interview at the US embassy in Ankara, Turkey, his application is stuck in the additional screening process known as “administrative processing”.

Ahmadi does not know why the visa is being held up now, after two years of living in the US and after another Iranian student in his research group at Clemson had her visa approved in late July.

But if the visa does not arrive soon, he will have to defer enrollment. This means being a year behind on courses only offered in the fall semester, losing a teaching position and missing his November PhD exam day. “It will ruin all my plans,” Ahmadi said.

To apply, an Iranian citizen must visit a US embassy in a different country, usually in the UAE, Turkey or Armenia. The wait for approval can be expected to last long past the US state department’s estimates.

“There’s no way you can count on anything, there’s no way of knowing when you’ll be approved,” said Leila Mansouri, the DC chapter president of the Iranian American Bar Association.

Mansouri said she has noticed the visa application process taking longer this past year, and knows of some recent cases where administrative processing has taken a year, even though the state department said it usually takes 30 days.

The situation is a far cry from the late 1970s and early 1980s, when Iran was for eight years the top sender of students to the US, according to the Institute of International Education.

That rate has declined dramatically from the peak in 1979, when 51,310 students from Iran were studying in the US, though it is rising back up slowly, with 11,338 students from Iran from 2014 to 2015, the most in 26 years. The IIE said these students contributed $323m to the US economy in 2014.

Students like Ahmadi are attracted to the US because of its top-ranking research facilities and funding opportunities, but the visa process is wearing on him and his peers.

About 170 of them gathered on the Telegram instant messaging app to discuss tips on how to study for graduate school qualifying exams but are now discussing their visa woes.

They have collected and analyzed their own data to try and predict when they could expect visa approval, but the results are predictably murky. Of the 170 students, 56 people have already missed the start of their classes.

“American governments say they differentiate between the Iranian government and Iranian people, but this specific case shows that is not true at all,” said Reza Marashi, research director at the National Iranian American council.

The state department said the US welcomes people from across the globe and that “academic cooperation increases cultural understanding and contributes to mutual progress in research and knowledge”. But it did not explain why these students are facing such a long delay.

Most of the Telegram group applied through the US embassy in Ankara, which directed the Guardian’s inquiries to Consular Affairs in Washington DC. The day after the Guardian contacted the US embassy in Ankara, 50 students received updates on their cases.

But the updates came too late, as most of the students had already deferred their admission or are worried they still will not get their visas before their individual deadlines to be at school.

While they wait, students are living with friends to avoid renting apartments that they will have to move out of abruptly.

“We don’t know what we’re doing and this is killing us,” said Amin, who asked that his real name not be used out of fears of retribution from the US or Iranian government.